The invention relates to a device for nondestructive testing of pipes made of ferromagnetic steel using magnetic or magnetic-induction test procedures.
Devices of this type have of a magnetizing yoke which transmits the magnetic flux contactless into the pipe and at least two magnetic-field-sensitive scanning probes implemented as GMR sensors as well as an evaluation unit.
Magnetic and magnetic-induction test procedures, for example the conventional magnetic leakage flux test, are used with pipes made of ferromagnetic steel to detect, in particular, longitudinal, transverse or inclined discontinuities, for example cracks, near the surface which cannot be detected at all or only with great imprecision with other test procedures, which tend to be expensive and time-consuming.
This method can be used to detect, for example, cracks extending from the surface of the pipe into the material by at least about 0.3 mm (Nondestructive Evaluation, A Tool in Design, Manufacturing, and Service, CRC Press 1997).
For example, DE 10 2004 035 174 discloses the use of so-called GMR sensors (Giant Magneto Resistance) in magnetic flaw tests, which have a high field sensitivity at low frequencies, are quite immune from electrical interference and can therefore also be employed, unlike conventional Hall sensors or induction coils, at greater distances from the test surface.
A comparison between inductive sensors (coils) for flaw detection and GMR sensors shows that GMR sensors have a high sensitivity, a high signal level, a low noise level and a high lateral resolution due to their small dimensions.
The higher sensitivity accompanied by the smaller noise level are advantageous when testing pipes, in particular for detecting interior flaws. The novel GMR sensors are therefore necessary to increase the range of wall thicknesses that can be tested, while simultaneously improving reliability. In addition, the low noise level offers enhanced possibilities for the test strategy.
As a consequence of the high lateral resolution, a single induction coil must be replaced by a plurality of GMR sensors (e.g., 8 elements) in order to be able to cover the same test surface and hence attain the same test performance.
Typically, each GMR sensor, like conventional Hall sensors in existing test systems, is operated with a dedicated difference preamplifier. The downstream evaluation electronics must then be configured with multiple channels.
If the intended test does not require a high resolution, then the number of channels has until now been reduced, due to the sensor properties, by employing an additional processing stage in the electronics or later in the digital section of processing. The test system consequently becomes quite complicated and expensive.
The use of a dedicated preamplifier for each GMR sensor necessitates a large number of components and connections. This complexity is necessary, in particular, if the increased lateral resolution is taken advantage of. Disadvantageously, the overall dimensions of the test unit increase substantially, which may cause problems in confined spaces.
For example, if each of the 8 coils in a test head are to be replaced by 8 GMR sensors having the same test surface, then 64 preamplifier are required instead of the previously employed 8 preamplifiers. In addition, the total number of connections increases from 9 (8+1 common ground) to 128. Due to the small size of the sensors, it is difficult to install this large number of connections in the test head.
It is would therefore be desirable overcome the shortcomings of the prior art and replace a test head for a magnetic or magnetic-induction flaw test having inductive sensors with a test head having at least two GMR sensors in form of an array, and to also reduce the complexity of the mechanical and electronic hardware, while substantially maintaining or even improving the test surface and test performance. Adjustment of the spatial resolution would also be desirable.